a) Field of the Invention:
The present invention relates to a communication network for carrying out communication while establishing and maintaining a logical ring by a solicit process.
b) Description of the Related Arts:
In FIG. 1, there is shown one example of a conventional communication network. In this network, four stations having station codes 00 to 03 are linked via a bus 10. In this case, one example of a practicable protocol is token passing which is, for example, defined by the IEEE standard 802.4.
In the token passing protocol, establishment and maintenance of a logical ring are essential. The logical ring is a ring composed of stations participating in transmitting and receiving data. In FIG. 1, a logical ring 12 is shown by a broken line and a token is transmitted from a station with a larger station code to one with a smaller one on the logical ring 12. Each station possesses information TS, PS and NS for representing its own station, a previous station and a next station, respectively. For example, the station 01 has information TS=01, PS=02 and NS=00.
In the token passing protocol, a transmission right of information is called a token. After obtaining the token, the station transmits a solicit frame to other stations when the number of the tokens obtained reaches a predetermined value by counting from the previous transmission of the solicit frame. The IEEE 802.4 defines a solicit .sub.-- any as the solicit frame. The solicit .sub.-- any is sent From a token hold station (the station holding the token) to all the other stations. Of the stations having received the solicit .sub.-- any, any station capable of responding does so. The token hold station determines a new next station which responded to the solicit .sub.-- any, is positioned in a token rotation direction along the logical ring, and has the station code closest to the token hold station. The token hold station transmits the token to this new next station. As a result, the station which responded to the solicit .sub.-- any joins the logical ring 12. In transmitting the token, for example, if the token is broken down on the bus 10 by noise or the like, since the next station cannot receive the token, the token is sent again. That is, the token is repeatedly transmitted within the limits of a predetermined number.
When no station responds to the solicit .sub.-- any, the token hold station considers that the logical ring 12 is composed of only its own station now and operates as follows. That is, after a bus .sub.-- idle.sub.-- timer expires, the token hold station sends a claim .sub.-- token a predetermined number of times to the other stations. The claim .sub.-- token is a frame claiming the token. After transmitting the claim .sub.-- token a predetermined number of times, the station having transmitted the claim .sub.-- token produces the token. Through a check on an access class the station having produced the token transmits the solicit .sub.-- any again. When no response against the solicit .sub.-- any is made, the claim .sub.-- token transmit station deems that the network condition is a sole.sub.-- active.sub.-- station, that is, only its own station participates in the communication and it stops the solicitation. This station proceeds to an idle state (awaiting a call or address to its own station) and thereafter awaits receipt of an effective call from another station.
Hence, for example, even in a case where the next station fails into a state that the station cannot receive the token because of power off or the like, the token hold station recognizes this situation by sending the token again to execute the solicitation of another station, enabling the maintaining of the logical ring 12.
However, when a problem occurs in the token hold station and it falls into a transmit-possible and receive-impossible state, conventionally, the whole network is driven into a transmit-receive operation stop.
When falling into the transmit-possible and receive-impossible state, the token hold station cannot receive any responses from the other stations to the solicitation. Hence, this station carries out an operation as if no station responds to the solicit .sub.-- any. As a result of this operation, the token hold station moves to the idle state. However, since the token hold station cannot receive any transmission from the other stations, the token hold station cannot return to the communication state. Further, the other stations except the token hold station are cut off from the transmission of the token and proceed to the idle state by a count-up or the like of a timer. Accordingly, the whole network becomes quiet. In order to restore communication from this state, it is necessary to initialize the stations except the faulty one and thus a manual operation is required.